It is known, for example from U.S. Pat. Nos. 1,531,130 and 1,955,628, as well as from British Pat. No. 141,877 and German patent document No. 3,025,896, to employ a fitting with a spark plug to form a firing chamber separate from the respective cylinder chamber. This firing chamber is shaped to facilitate ignition of the fuel-air mixture that powers the engine. The electrodes of the spark plug are provided in this firing chamber, which communicates through ports with the cylinder. Thus when a fuel/air mixture is ignited in the firing chamber it will shoot out through the port or ports as a spike or spikes of flame that effectively ignite the gases in the cylinder chamber.
Such ignition of the gases in the cylinder chamber is quite effective when the engine is idling, particularly when a fairly lean mixture is being used. The torch action is much more effective than a simple spark in igniting a fuel/air mixture, especially in an engine that does not scavenge thoroughly, or as mentioned where a very lean mixture is being used to spare fuel.
A problem with these systems is that they fire very rapidly. The tiny amount of the fuel/air mixture that is forced on the compression stroke into the firing chamber. Thus if a low-octane fuel is used, for example, it is possible to misfire and stall unless the engine timing is perfect.